The present invention relates to the control of stud-welding apparatus for automatic attachment of welding studs and in particular to the control of apparatus using the technique of arc ignition.
In arc ignition stud-welding apparatus, a stud is initially presented to contact a workpiece to which the stud is to be welded, and a current, often low, is then passed through the stud to create a short circuit and the stud is then withdrawn a predetermined distance (the "lift stroke") to create an arc which is used for the welding process. When the arc is ignited, the welding current is increased so that a pool of molten metal is formed at the welding point on the surface of the workpiece and the stud. The operation is completed by a plunge stroke which applies the stud to contact the workpiece through the molten pool. Subsequent solidification of this molten metal secures the stud to the workpiece.
In such apparatus the stud is carried by a stud carrier which is lifted from the workpiece as the arc is initiated and plunged back to the workpiece when full power has produced the requisite welding pool. In many common forms of apparatus, the lifting force is applied electromagnetically and a mechanical device such as a spring urges the stud-holder in the plunge direction, so that the electromagnetic lift has to operate against the plunger spring. Unfortunately the stud thus tends to complete the plunge stroke at high force and speed, and the impact as the stud contacts the workpiece may splatter molten metal around the weld, which is inefficient, unsightly and even dangerous.
There have been a number of proposals to damp the plunge stroke so that the contact of stud and workpiece is achieved with less force and at lower velocity. Typical proposals are described in U.S. Pat. Nos. 3,242,307 and 4,562,328. The latter discloses mechanical damping only the latter part of the plunger stroke, the initial part of the plunger stroke being free travel caused by the plunger spring without application of damping.
It is an object of the present invention to provide improved control of stud-welding apparatus comprising a stud-holder wherein an arc is initiated by passing a current through a stud held by the stud-holder in contact with the workpiece, the arc is established by lifting the stud from the workpiece and applying full power to the arc to form pools of molten metal on the stud and on the workpiece, and welding contact ("impact") finally made at the end of the plunge stroke wherein the plunge stroke is controlled without mechanical damping.